Thermoset molding compositions known in the art are generally thermosetting resins containing inorganic fillers and/or fibers. Upon heating, thermoset monomers initially exhibit viscosities low enough to allow for melt processing and molding of an article from the filled monomer composition. Upon further heating, the thermosetting monomers react and cure to form hard resins with high stiffness. Some thermoset molding compositions derived from poly(arylene ether) and alkenyl aromatic monomers exhibit a substantial drop in viscosity when heated above approximately 60° C. At molding temperatures, often well above 60° C., the drop in viscosity may contribute to poor glass carry and non-uniform glass bundle distribution of molded thermoset compositions containing glass or other fillers.
One industrial use of thermoset compositions is the molding of automotive body panels. These panels preferably exhibit high dimensional stability and a smooth as-molded surface. It is also preferred that the dimensions of the molded parts conform closely to those of the molds used to prepare them.
Thermoset compositions based on unsaturated polyester resins and styrene are known to exhibit reduced shrinkage and improved surface properties when they incorporate a so-called low-profile additive, such as a polymethacrylate copolymer. See, for example, V. A. Pattison et al. J. Appl. Poly. Sci, volume 18, pages 2763–2771 (1974). Although known low-profile additives improve the performance of the polyester thermosets, there is a need for compositions exhibiting further improvements, particularly in surface characteristics.
U.S. Pat. No. 6,352,782 to Yeager et al. describes thermoset compositions comprising poly(arylene ether) resins that have been capped with ethylenically unsaturated groups. These compositions exhibit desirable properties including high glass transition temperatures and low coefficients of thermal expansion. However, low-profile additives known for polyester thermosets are ineffective in the poly(arylene ether)-containing compositions.
Some thermoset compositions exhibit tack in their curable form. Curable resins exhibiting tack makes them difficult to handle, adhering to items such as equipment, clothes, containers, etc. Tackiness may result in a uniform mass of resin in a storage container rather than individual pieces or pellets in a free-flowing form. Special packaging methods must be used to avoid such problems in the handling and transfer of tacky resins.
There remains a need for thermoset compositions exhibiting reduced shrinkage on molding and improved surface characteristics. There also remains a need for curable thermoset compositions having reduced tack. Finally, there remains a need to maintain sufficient viscosity of thermoset compositions during molding at elevated temperatures in order to provide good glass carry and uniform glass bundle distribution during molding.